1. Field of the Invention
The present invention relates to a suspension spring for geophones or seismometers.
2. Description of the Prior Art
Geophones are devices which sense motion by means of suspending an inertial mass structure from a rigid, fixed reference supporting structure. Typically, the mass is a coil suspended by springs in magnetic field, one spring attached at each end of the coil. The effect of the springs is to position the coil within the magnetic field and to give the "suspension system" a pre-determined resonant frequency.
The springs most commonly used in present-day geophones are spider springs which consist of an inner ring and an outer ring with spokes or legs connecting the two rings. Generally, three legs are utilized inasmuch as this is considered the most stable arrangement.
The object of the geophone is to sense movement from only one direction and, therefore, it is desirable to eliminate or minimize the effects of any motion from a direction not parallel to the axis of movement of the suspended coil within the geophone. However, the nature of the spring-suspension system results in a component of vibration along the axis of movement of the suspended coil when the geophone is moved laterally. This produces an undesired or false signal which is referred to in the art as "spurious resonance".
Inasmuch as the geometry and mass (coil) for a given model of geophone are constant within manufacturing limits, the spurious resonance is also constant. Therefore the frequency of this spurious resonance can be raised or lowered by changing the geometry of the suspension springs. This characteristic has provided a means for segregating or eliminating false signals which are generated by lateral movement of the geophone. Usually, the problem is handled by raising the frequency of the spurious resonance to a level well above the frequency spectrum of interest for the geophone. The present state of the art normally requires a ratio of about 30 to 1 of the spurious resonance to the natural frequency of the geophone.
The most common method of achieving the desired ratio of spurious resonance to natural frequency is to shorten the spring legs, thereby raising the frequency of the spurious resonance. While this is an effective method, it has undesirable side effects.
The legs of prior art geophone springs have been of a rectangular cross-section and curved along their lengths between the junctures with the inner ring and outer rings of the spring. The spring is "preformed", that is the inner ring is offset or displaced from the outer ring such that when the mass (coil) is suspended between two such springs, the inner ring, legs, and outer ring of each spring lie in the same plane. The preforming causes a lateral bending force to be exerted upon each spring leg. Due to the characteristics of curved beams, which the spring legs represent, forces acting thereon will cause inner fibers of the beam to be stretched more than outer fibers. Consequently, in the case of the spring legs, the yield point of the material of the legs will be reached first for the inner fibers with the consequence that such fibers will be stretched or distorted out of shape. This can cause the spring to be non-linear and result in the production of a distorted signal from the geophone. This effect is increased when the legs are relatively short and the preforming or offset is large as generally occurs in low frequency geophones with high spurious resonance.
Accordingly, prior to the development of the present invention, geophone springs consisting of an inner and outer ring and legs joining the rings have been characterized by a nonlinearity and the existence of a bending force which distorts the harmonic motion of the spring system and the resulting signal generated by the geophone. Therefore, the art has sought an effective, simple geophone spring which minimizes the problem of harmonic distortion.